Chemical-mechanical polishing (CMP) is a method used in semiconductor processing to planarize step-like features on a wafer. With CMP, a wafer is pressed (upside down) against a rotating polishing pad in the presence of a chemically corrosive slurry. The action of the slurry and the rotary motion combine to remove a desired amount of material from the wafer and achieve a planar surface.
The main goal for a typical CMP process is a high degree of flatness or planarity. Planarity both locally (for closely spaced features) and globally (i.e. uniformity across the wafer) are very important. This is made difficult by the fact that wafers are often not flat to begin with, and during processing, features of various sizes and densities are created across the wafer.
Commercially available polishing pads come in a variety of hardness types. Soft pads can more easily conform to the different features on the wafer and tend to achieve global planarity at the expense of local planarity, while hard pads conform less and tend to achieve local planarity at the expense of global planarity. Soft pads also tend to polish away material more slowly, given the same speed and pressure as a hard pad, but produce less scratching than a hard pad. Soft pads also provide a better vehicle than hard pads for delivering slurry to the polishing site, as the slurry can soak into the soft pad material.
Composite pads have been created to attempt to combine the best features of soft and hard pads. Two examples are "sandwich" types which use vertical stacking of hard and soft layers (see U.S. Pat. No. 5,212,910 to Breivogel, et al.), and "distributed" types which attach hard pieces to a soft support layer (see U.S. Pat. No. 5,230,184 to Bukhman). However, these types of composite pads tend to degrade easily over time as the pads wear, the soft material tends to lose its elasticity, and the pad becomes loaded with polish residuals and slurry. Pad conditioning (scraping away the top layer) thus is required more frequently. As a result, pad life is further shortened and process stability and reliability suffers.
Thus, there remains a need for a composite polishing pad that provides local and global planarity, extended pad life, and good slurry delivery.